Last year, I started giving pretests at the beginning of each unit. The students were upset because they didn’t know many of the answers, even though I explained I didn’t expect them to know everything and the pretest wouldn’t count as a grade. Are there other ways to find out what students know about a topic?
—Cheri, Bangor, Maine
One of the most important factors influencing students’ learning is not the activities you plan or the materials you use but what students already know about a topic. In her recent Edutopia blog Are You Tapping into Prior Knowledge Often Enough in Your Classroom? Rebecca Alber describes the research on the value of accessing the knowledge, skills, and experiences students bring to a learning unit.
Reflecting on my own experience with middle-schoolers, I probably did not “tap” enough at first. I was so excited about the unit topic and the great activities that students could do. But a student changed my mind. She turned in a test with tears in her eyes. “I know a lot about this, but you didn’t ask the right questions.” I asked her what she meant (the topic was marine invertebrates) and she described her summers at her grandparents’ cottage on the beaches of Florida and the extensive shell collection she had at home. I realized that if I had known that in advance, I could have asked her to share her experiences with the other students, many of whom had never seen the ocean (I should have known that ahead of time, too).
Alber mentions some activities that can help students access their prior knowledge and previous experiences. These can easily be adapted to science learning and included in science notebooks, as alternatives to traditional pretests. These are more open-ended and can tease out things that you may not have considered:

• KWL charts are three-column graphic organizers on which students note what they already (K)now about a topic, what they (W)ant to know, and finally what they (L)earn about a topic. The K and W columns can provide information prior to instruction on students’ knowledge and interests. If your students don’t know what they want to know (for the W column), ask them what they (W)onder about instead.

• Using a visual as a prompt, ask students to list what they know about a topic or to generate a list of related words. As with a KWL chart, students can include what they’ve learned about a topic from a variety of sources, including what they may have learned outside of school

• On a list of key vocabulary or concepts, ask students to put a plus sign next to those they’re comfortable with, a check mark next to those they’ve heard of but are not sure about, and a question mark next to those with which they are completely unfamiliar.

• Alber describes “ABC Brainstorming” in which students think of 26 words related to a topic from A to Z (I’d suggest students use words starting with “ex” for X). I used a similar version but instead of A-Z, students used the letters of their names or the letters in a related term such as “photosynthesis.” Student enjoyed sharing their lists. It’s interesting to do this again at the end of the unit to see if students respond differently or in greater detail.

Assessing students’ prior knowledge can also identify misconceptions or incomplete understandings. Page Keeley has created a series of books on Uncovering Student Ideas in Science. The “probes” in these books are brief activities that help teachers identify students’ preconceptions or misconceptions about a topic.
From a practical standpoint, if you find most have some familiarity with a topic, you won’t need to spend a lot of time reteaching the basics, other than perhaps a brief review. You can develop more in-depth activities and topics that build on the students’ knowledge and experiences. However, if students do not have the background knowledge and skills you expected, you’ll need activities that introduce students to fundamental concepts and processes.
Finding out a students’ prior knowledge can also be helpful in differentiated instruction. Once you identify what experiences and knowledge students have, you can plan activities for those who need basic instruction and for those who are ready for more advanced work.
I would be frustrated when students claimed they were unfamiliar with a topic I knew they’d covered in previous classes.  I found teachers in other years may have used different vocabulary. These activities can refresh students’ memories, helping them realize they knew more than they (and I) thought.
 
Photograph: http://www.flickr.com/photos/rongyos/2686415336/